As illustrated in FIG. 23, a synchronous rectification type power circuit includes a high-side switch (101) for supplying a voltage (Vdd) on a high potential side to an output terminal (103) and a low-side switch (102) for supplying a voltage (Vss) on a low potential side to the output terminal (103), and these switches are alternately turned ON/OFF to output a desirable voltage to the output terminal (103). An output inductor (106) and an output capacitance (107) are connected to the output terminal (103). In order to prevent an inductor current (IL) from flowing reversely to the low potential side in a light load, resulting in a reduction in an efficiency, there is a DCM mode (Discontinuous Conduction Mode) for turning OFF the low-side switch (102) to provide a period in which both of the switches are turned OFF. An optimum control in the DCM mode, that is, a timing for turning OFF the low-side switch (102) is equivalent to a moment that the inductor current (IL) reaches zero. For this reason, there are proposed various control methods for turning OFF the low-side switch (102) in an optimum timing. In an output voltage of the output terminal (103), ringing is generated by a parasitic capacitance possessed by an inductor or the like after the low-side switch (102) is turned OFF and an output current then reaches zero. There is also a possibility that the voltage influenced by the ringing might exceed a power supply voltage depending on an input/output voltage or a characteristic of the inductor. For this reason, a countermeasure against the ringing of the output voltage is required for the control in the DCM mode, which is not sufficient.